It is regulated that exhaust gas emitted from internal-combustion engines of automobiles contains NOX, etc. (referred to as “specific gas” herein) concentration less than a defined amount. Therefore, the exhaust gas is monitored by a gas concentration sensor mounted on the automobile. The gas concentration sensor is provided in a solid electrolyte device. The solid electrolyte device is heated by heaters so that its temperature is raised to an active temperature of the specific gas. If the heater performance is degraded and the solid electrolyte is not heated enough or is overheated, the specific gas cannot be maintained at the active state to make it difficult to precisely detect the specific gas.
Therefore, it is desired to detect the degradation of the heater performance. As such detectors, a scheme is proposed where the initial state and the present state of the heaters are compared to detect the degradation of the heater performance, as disclosed in Japanese Patent Laid-open Publication No. 2002-155796. In this scheme, the initial states and the present states of the gas concentration sensor and the heaters are compared. If they are significantly changed, it is determined that the performance of the heaters is degraded.
This scheme, however, monitors the internal resistances of the actually used gas concentration sensor and heaters as their present internal resistances. The internal resistances of the actually used gas concentration sensor and heaters can be significantly changed, and therefore the present internal resistances cannot be reflected or detected and the correct comparison with the initial internal resistance of the heaters are not achieved.